1. Field of the Invention
The present invention relates to a light-emitting device, more particularly, to a flat plate type light-emitting device such as a flat fluorescent screen to be used, for example, as a backlight for a liquid crystal display device, and a process for its production.
2. Discussion of Background
As a light-emitting device employing an airtight container, a flat fluorescent screen, fluorescent tube or plasma display panel (PDP) employing gas discharge, a cathode ray tube (CRT) utilizing electron beam, a vacuum fluorescent display (VFD), a field emission display (FED) an electroluminescent display (EL) including organic EL, or an electric lamp employing emission by heating, is, for example, known. One having a single display dot is called a single tube, and one having plural display dots is called a multi tube. For a flat fluorescent screen to be used as e.g. a backlight for a liquid crystal display device, in order to make the device thin, it is preferred to employ an airtight container using flat plate type front and rear substrates.
For a flat plate type light-emitting device employing gas discharge, like a flat fluorescent screen, electrodes, a dielectric layer and a phosphor layer are formed on the surface of glass plates constituting a front substrate and a rear substrate, and then the joint portion between the front substrate and a spacer component and the joint portion between the rear substrate and the spacer component, are airtightly sealed by means of a glass sealing material such as a lead-containing frit glass in a state where the front substrate and the rear substrate are maintained to have a certain distance by the spacer component (spacer), to form an airtight container. Then, the container is evacuated through an exhaust pipe attached to the airtight container or through a hole formed on the glass substrate to bring the interior to a predetermined vacuum degree, and then a discharge gas will be sealed in to a prescribed pressure. After sealing the discharge gas, the exhaust pipe attached to the airtight container is cut, and the hole formed in the substrate for evacuation, is sealed by glass and a glass sealing material.
Further, instead of the above-described procedure, there may be a case wherein after forming electrodes, a dielectric layer and a phosphor layer on the surface of glass plates constituting the front substrate and the rear substrate, the front substrate, the rear substrate and the spacer component are dried in vacuum in a vacuum chamber, and then, in a state where the interior of the chamber is substituted by a prescribed discharge gas atmosphere, the joint portion between the front substrate and the spacer component, and the joint portion between the rear substrate and the spacer component, are airtightly sealed with a glass sealing material, to form an airtight container having the discharge gas sealed in.
In order to let the flat fluorescent screen thus formed, emit light efficiently, it is necessary to set the discharge distance to be constant, which is determined by the distance between the front substrate and the rear substrate.
For sealing of an airtight container for a light-emitting device employing gas discharge like a flat fluorescent screen, it is common to employ a lead-containing low melting point glass as a glass sealing material (JP-A-2003-522369), and airtight sealing is carried out at a temperature of from 400° C. to 550° C., which is a temperature of at least the softening point of the glass for sealing. Other than the lead-containing low melting point glass, a bismuth-containing low melting point glass or one formed by a laminate of the lead-containing low melting point glass and the bismuth-containing low melting point glass, may also be used. The color temperature of white color of the flat fluorescent screen which is airtightly sealed by means of a lead-containing low melting point glass, tends to be low due to deterioration of the phosphor by heat. Among phosphors for three primary colors, a blue-emitting phosphor is particularly susceptible to deterioration by heat, and a study is being made to complement the deteriorated portion by increasing the amount or the coating area of the blue-emitting phosphor, or to convert it to a material system hardly susceptible to heat deterioration by improving the composition of the blue-emitting phosphor (JP-A-2003-82344, JP-A-2003-82345) or to apply a coating on the surface of blue-emitting phosphor particles (JP-A-2003-82343, JP-A-2003-41247, JP-A-2003-41248).
Further, a study is also being made to suppress deterioration of a blue-emitting phosphor by using a dry gas for the atmosphere in a heating step such as a firing step for phosphor layer, a preliminary firing step for low melting point glass, a sealing step or an evacuation step (JP-A-2003-109503, JP-A-2002-367522). Further, a study is also being made on the composition of a discharge gas to prevent deterioration of a blue-emitting phosphor in an aging step (JP-A-2001-35380, JP-A-2001-23525). Furthermore, as a sealing agent for sealing glass to be used for sealing a vacuum fluorescent display or the like, a sealing composition is known which comprises a curable silicone resin and a refractory filler (JP-A-2001-207152).
Namely, for sealing of an airtight container for a light-emitting device employing gas discharge, like a flat fluorescent screen, a lead-containing low melting point glass has heretofore been used as a sealing material to carry out airtight sealing at a temperature of from 400 to 550° C. In the sealing employing a lead-containing low melting point glass, the phosphor undergoes heat deterioration to cause a decrease in the color temperature or the luminance, in a preliminarily firing step for low melting point glass or a sealing step. A blue-emitting phosphor is particularly susceptible to heat deterioration, and many studies have been made such as to increase the amount of the blue-emitting phosphor, to increase the heat deterioration resistance by improving the composition of the phosphor or by coating the surface of phosphor particles, and to carry out heat treatment in a dry gas atmosphere to avoid deterioration of the blue-emitting phosphor in a heating step such as a firing step for phosphor layer, a preliminary firing step for low melting point glass, a sealing step or an evacuation step, but adequately satisfactory color temperature characteristics have not yet been obtained. Further, the conventional sealing step includes a heating step at a temperature of from 400 to 550° C., whereby there is a problem such that the energy consumption is high, or the operation time is long, leading to a high cost. Further, the conventional glass sealing material contains a lead component to lower the melting point, but the hazardous nature of lead has been pointed out, and it is desired to develop a light-emitting device having an airtight container sealed by means of a sealing material not containing a hazardous component such as lead or cadmium. As a sealing material not containing a hazardous component such as lead or the like, a material such as phosphate glass is available, but such a material has a problem that the bond strength at the sealed portion is weak.